In the Fibroid Growth Study(FGS) we collected a variety of data, such as MRI measurements of fibroid volumes over time (up to 4 time points), gene expression microarray data obtained on tumors from women who opted for surgery, etc. We are presently analyzing data obtained from that study. As a follow-up to our recent publication (Peddada et al., PNAS, 2008), in this project we evaluated the short term growth spurts and changes in growth rates of fibroids using a sample of 101 fibroids that were measured at enrollment, 3, 6, and 12 months. A fibroid is said to experience a growth spurt in a particular time interval if (i) its volume increased at a rate of at least 30% per 3 months during that interval and (ii) the difference between that rate and the growth rate in each of the other two intervals is at least 30%. For example, suppose a fibroid grew at the rates of 45%, 25% and 7% (per 3 months) in the three time intervals. Then this fibroid satisfies (i) but not (ii) and hence according to our definition it did not experience a growth spurt. However, a fibroid with growth rates 1%, 35%, 2% experienced a growth spurt in the second time interval. Growth spurts were observed in 37 of the 101 fibroids, a prevalence nearly tenfold higher than that attributable to potential measurement error. An overall measure of short-term change in growth rate for each fibroid was obtained by calculating the variance of the three interval-specific growth rates. Interestingly, fibroids within the same woman varied significantly in their short-term change in growth rate. Neither the woman-specific factors, such as, age, race/ethnicity, parity, body mass, nor the fibroid position in the uterus explained the differences among fibroids within the same woman. However, large fibroids (>5 cm diameter) had a smaller short-term change in relative growth rate than smaller fibroids. These findings may be clinically relevant.